A common way of achieving fault-tolerance in computer and telecommunications systems is to duplicate the fault-prone unit and operate the system with one of the duplicate units active and performing system tasks, while the other duplicate unit is either inactive or operating in lock-step with the active unit and standing by to take over the performance of system tasks upon the failure the active unit. For example, in telephony switching systems, it is common to duplicate the switching fabric and/or the system control computer (including the computer memory and its contents) and to operate the duplicate units in active-standby mode.
In the Case of duplicate, active-standby, memories, proper system operation requires that the contents of the memories be identical. Initially, this is achieved by loading the same contents into both duplicate memories at initialization. During system operation, synchronization of the duplicate memory contents is maintained by performing all write operations on both memories, so that the duplicate contents of both memories change identically.
During system operation, following a failure of the active memory, substitution therefor of the standby memory, and repair of the formerly-active memory, it is necessary to bring the repaired, now standby, memory into synchronism with the now active memory in order to again achieve fault-tolerance. This requires the contents of the two memories to again become, and to remain, identical. This means that the repaired memory must be populated with the contents of the active memory. In the prior art, population of the standby memory with contents of the active memory is achieved by serially reading the contents out of the active memory and writing the read-out contents into the standby memory, and in the meantime also writing into the standby memory any changes being made to those contents of the active memory that have already been copied into the standby memory. An illustrative system of this type is disclosed in U.S. Pat. No. 3,864,670.
This scheme for achieving synchronism between duplicate memories has disadvantages, however. On the one hand, if the system processor is used to copy the contents of the active memory into the standby memory, processing power for this job is taken away from system tasks, and system performance is adversely affected. On the other hand, if a separate controller is provided to perform the copying, or if the memories perform the copying operation autonomously, between system-processor accesses of the active memory, it may take a long time to achieve memory synchronization, particularly in systems that are performing memoryintensive tasks. But the system is not fault-tolerant without memory synchronization, making it imperative to achieve synchronization as soon as possible.